The present invention relates to a method for controlling vaporized fuel generated from the fuel tank of an automobile and more particularly to a method for purge control of the vaporized fuel stored in a charcoal canister. In a conventional vehicle, for the purpose of preventing vaporized fuel or fuel vapor in the fuel tank from being emitted outside, an evaporative emission control system is widely used. In the evaporative emission control system, fuel vapor is guided to a charcoal canister and is adsorbed therein. The adsorbed fuel vapor is sucked into the intake system of an engine and then it is burned together with the mixture gas in the combustion chamber. The process of fuel vapor being sucked into an engine is called "canister purge."
However, generally speaking, this canister purge causes a deviation as much as an amount of fuel vapor discharged into the intake air passage in air-fuel ratio because the air-fuel ratio is determined depending on the amount of intake air. To solve this problem, for example, Japanese patent application laid open No.1988-18175 discloses a technology to control a canister purge without having an effect on the air-fuel ratio.
This patent application proposes:
When fuel vapor is supplied to the intake air passage at an operational point, the allowable amount of fuel vapor is judged and based on the judged amount and the supplied amount of fuel vapor is adjusted to the allowable amount. Throughout all operations in the same way the supplied amount is adjusted to the allowable amount. As a result of this it becomes possible to discharge fuel vapor into the intake air passage without having an effect on the feedback control.
As well known, in an air-fuel control for the conventional engine, a learning control system has been introduced so as to correct a deviation of air-fuel ratio derived from production scatterings or deteriorations in such components as an induction air flow sensor, a fuel injector and other components as quickly as possible and further so as to keep air-fuel ratio at a desired value even when the engine operating condition is largely changed. That is to say, at the previous running of engine, a deviation of the centerline for so-called LAMDA control coefficient is memorized on a map and at the present running, fuel injection amount is corrected by referring the deviation memorized on the map, whereby air-fuel ratio is controlled properly.
In a prior art, the amount of canister purge is so designed to be fixed at a set value regardless of the loading condition of canister with fuel vapor that when a canister purge is performed in conditions of a canister fully loaded with fuel vapor, such as under a high ambient temperature condition or at a high altitude running, air-fuel ratio deviates to the rich side and on the other hand, when a canister purge is done in a condition of a canister less loaded with fuel vapor, air-fuel ratio deviates to the lean side. As a result of this, the air-fuel ratio feedback control recognizes above deviations as those of the standard value of an air-fuel ratio feedback correction coefficient .differential. and the learning value which is stored in a map is updated to the rich or lean sides with the new standard value of .differential. which has been corrected by those deviations. This updated learning value is used even when a canister purge is not performed, so that air-fuel ratio becomes inappropriate and consequently poor driveability and inferior emissions are caused.